1. Field of the Invention
This invention relates to a method for forming an LED device, and more particularly to a method for forming an LED device with a metallic substrate.
2. Description of the Prior Art
The working of an LED is to emit light by a current flow forward into an active layer of a semiconductor device. FIG. 1A shows a popular LED structure. In FIG. 1A, a P-type bonding pad 126 and an N-type bonding pad 118 are respectively formed above and below an LED epitaxial layer 110, which is formed on a substrate 117. The LED epitaxial layer 110 comprises an N-type semiconductor layer 112, an active layer 114, and a P-type semiconductor layer 116. A current 130 is injected from P-type bonding pad 126, passed through the P-type semiconductor layer 116, the active layer 114, the N-type semiconductor layer 112, and the substrate 117, and finally flowed out of the N-type bonding pad. Light is emitted while the current density in the active layer 114 reaches a threshold value. The N-type semiconductor layer 112, the active layer 114, and the P-type semiconductor layer 116 totally are called the LED epitaxial layer 110.
The structure mentioned above is a vertical structure, so the chip process is simple. Furthermore, the bonding step is easy in package process and the product yield is high because only one wire bonding is needed. Moreover, high power LEDs, which gained great attention lately in lighting applications, drive a larger current to generate a higher light output, and the larger current generates more heat. Therefore, the operating temperature of the high power LED structure is higher, and the lifetime and the reliability of the LED can be substantially decreased. Furthermore, the luminous efficiency is also reduced while the operating temperature of the LED structure rises. Hence, the high power LEDs must have a structure having a better thermal dissipation effect. Nevertheless, the conventional LED structure does not have a good thermal dissipation effect.
FIG. 1B shows a schematic diagram of a structure of GaN-based LED different from the structure as shown in FIG. 1A. The material of the substrate is sapphire, which is not conductive, different from the conductive substrate in FIG. 1A. In FIG. 1B, there are an N-type GaN semiconductor layer 132, an active layer 134, and a P-type GaN semiconductor layer 136 which are formed on a substrate 137 in sequence. Due to the substrate is not conductive, a P-type bonding pad 148 and an N-type bonding pad 146 must be formed on the same side of the substrate. Hence, a partial surface of the GaN epitaxial layer 140 (comprising the N-type GaN semiconductor layer 132, the active layer 134, and the P-type GaN semiconductor layer 136) is etched to expose a partial surface 133 of the N-type GaN semiconductor 132, and the N-type bonding pad 146 is formed on the surface 133. In addition, the electrical conductivity of P-type GaN semiconductor layer 136 is poor, and a semi-transparent contact layer 139 has to be formed on the P-type GaN semiconductor layer 136 to enhance the current spreading.
The aforementioned structure is not a vertical structure. Compared with the structure as shown in FIG. 1A, the chip process is more complicated. Moreover, such structure has two pads on its front surface, so the package process is more complicated and the overall yield can be lower. Besides, the thermal dissipation effect and the electrostatic discharge effect are not satisfactory due to the substrate is not conductive.
In the conventional arts, a structure as shown in FIG. 1A is further formed a permanent substrate on an opposite surface to the substrate and then the substrate is removed. Therefore, the material of the permanent substrate can be appropriately selected, such as a material having better optical transparency, better electrical conductivity or better thermal dissipation effect. However, the permanent substrate is formed on the LED epitaxial layer by wafer bonding, and the wafer bonding must be conducted under high temperature and high pressure conditions, thus the yield of the LED is relatively low.